Radio frequency identification (RFID) tags are used in a variety of different environments, including identification of goods in a warehouse as well as for environmental sensing. The energy storage capability of a tag limits a reading range between the tag and a reader. As the processing capability of tags increases, the consumption of the limited stored energy also increases, thereby further limiting the reading range. Furthermore, the transmission medium between the tag and the reader can limit the range. For example, a tag embedded in an automobile to facilitate manufacturing, or a tag embedded in an article of clothing, will include material between the tag and the reader other than free-space air.
Typically, a tag stores energy with a battery, which increases the cost and size of the tag. Alternatively, near-field techniques have been used, however such techniques are limited to short distances (e.g. on the order of centimeters) and rely on inductive or capacitive coupling. RFID tags require an efficient way of being powered over a long distance in an environment without tethering, without using dangerously high levels of power, or imposing undue restrictions on the placement of charging stations used to power the RFID tags.